System and integrated process for liquid natural gas production

ABSTRACT

A system and method for producing liquid natural gas (LNG) from a natural gas stream is presented. The system includes a moisture removal device and compressor for removing moisture from and compressing the natural gas stream. The low moisture compressed natural gas stream is cooled in a heat exchanger to discharge a cooled compressed discharge stream. A multi-phase turbo expander provides for further cooling and expansion of the cooled compressed discharge stream, generating an expanded exhaust stream comprising a mixture of a vapor comprised substantially of CH 4  and a LNG/ice/solid CO 2  slurry. The expanded exhaust stream is separated to generate a vapor stream comprised substantially of CH 4  and a liquid natural gas/ice/solid CO 2  slurry stream. Further separation of the liquid natural gas/ice/solid CO 2  slurry stream generates a liquid natural gas output stream and an output stream comprised substantially of ice/solid CO 2 .

BACKGROUND

The present disclosure relates to a system and process for liquid natural gas (LNG) production. More particularly, the present disclosure relates to a system and process for liquid natural gas production that integrates natural gas cleanup and refrigeration steps in a single process.

Conventional liquid natural gas production from pipeline quality natural gas typically utilizes a two-step process in which initially moisture and CO₂ are removed in a first processing step, or cleanup step, and in a subsequent second processing step, refrigeration is utilized to provide liquefaction. In an attempt to integrate these multiple process steps, previous attempts have provided for integration of the natural gas cleanup and refrigeration processes in the production of liquid natural gas. Typically, these attempts were based on processes in which initially a slurry, comprised of liquid natural gas, ice and solid CO₂, was produced during expansion in a nozzle and external refrigeration systems provided energy required for liquefaction. In general, minimal equipment integration is provided and thus a significant equipment footprint is required. This results in a costly system and process for the production of liquid natural gas.

In addition, these known LNG production processes, such as, for example, the above-described two step process, may be energy intensive as well as capital intensive.

Accordingly, it is desired to provide for an improved system and method for the production of liquid natural gas.

BRIEF DESCRIPTION

These and other shortcomings of the prior art are addressed by the present disclosure, which provides an improved system and method for the production of liquid natural gas.

One aspect of the present disclosure resides in a system for producing liquid natural gas (LNG) from a natural gas stream, comprising: a moisture removal device, a compressor, a heat exchanger, a multi-phase turbo expander, a separator, and at least one additional separator. The moisture removal device and the compressor remove moisture from and compress the natural gas stream and generate a low moisture compressed natural gas stream. The heat exchanger cools the low moisture compressed natural gas stream and generates a cooled compressed discharge stream. The multi-phase turbo expander expands the cooled compressed discharge stream and generates an expanded exhaust stream comprised of a mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry. The separator separates the expanded exhaust stream and generates a vapor stream comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry stream. The at least one additional separator separates the liquid natural gas/ice/solid CO₂ slurry stream and generates a liquid natural gas output stream and an output stream comprised substantially of ice/solid CO₂.

Another aspect of the present disclosure resides in a system for producing liquid natural gas (LNG) from a natural gas stream, comprising: at least one compression stage, at least one cooling stage and at least one expansion stage. The at least one compression stage and the at least one cooling stage are configured to compress and cool a natural gas stream and generate a cooled compressed discharge stream. The at least one expansion stage configured to expand cooled compressed discharge stream and generate an expanded exhaust stream. The at least one expansion stage comprising at least one multi-phase turbo expander in fluid communication with the cooling stage and the compression stage. The multi-phase turbo expander comprising: a housing; at least one rotating component disposed within the housing; at least one inlet disposed in the housing, wherein the inlet is configured to receive the cooled compressed discharge stream; and at least one outlet disposed in the housing, wherein the outlet is configured to discharge an expanded exhaust stream comprising a mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry.

Yet another aspect of the disclosure resides in a method for producing liquid natural gas (LNG) from a natural gas stream, comprising: providing an input natural gas stream; removing moisture, compressing and cooling the natural gas stream and generating a cooled compressed discharge stream; expanding the cooled compressed discharge stream in a multi-phase turbo expander and generating an expanded exhaust stream comprising a mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry; separating the expanded exhaust stream in at least one separator and generating a vapor stream comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry stream; and separating the liquid natural gas/ice/solid CO₂ slurry stream in at least one additional separator and generating an output stream comprised substantially of ice/solid CO₂ and a liquid natural gas (LNG) output stream.

Various refinements of the features noted above exist in relation to the various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may exist individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. Again, the brief summary presented above is intended only to familiarize the reader with certain aspects and contexts of the present disclosure without limitation to the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 is a block diagram of a system for liquid natural gas production, in accordance with one or more embodiments shown or described herein;

FIG. 2 is a block diagram of a system for liquid natural gas production, in accordance with one or more embodiments shown or described herein;

FIG. 3 is a schematic of a multi-phase turbo expander for solid, liquid and gas separation, in accordance with one or more embodiments shown or described herein;

FIG. 4 is a schematic of a cross-sectional view of a multi-phase turbo expander for solid, liquid and gas separation, in accordance with one or more embodiments shown or described herein;

FIG. 5 is a flow chart representing steps involved in an exemplary process for the production of liquid natural gas, in accordance with one or more embodiments shown or described herein; and

FIG. 6 is a comparative plot of liquid natural gas yield as a function of compression, in accordance with one or more embodiments shown or described herein.

DETAILED DESCRIPTION

The disclosure will be described for the purposes of illustration only in connection with certain embodiments; however, it is to be understood that other objects and advantages of the present disclosure will be made apparent by the following description of the drawings according to the disclosure. While preferred embodiments are disclosed, they are not intended to be limiting. Rather, the general principles set forth herein are considered to be merely illustrative of the scope of the present disclosure and it is to be further understood that numerous changes may be made without straying from the scope of the present disclosure.

As described in detail below, embodiments of the present disclosure provide systems and processes suitable for the production of liquid natural gas. As discussed in detail below, embodiments of the present disclosure comprise systems including one or more integrated multi-phase turbo expanders and refrigeration means capable of operation with multi-phase flows (gas, liquids and solids). The system includes cooling a gas stream to form liquid natural gas and natural gas impurities, including CO₂ gas, solid CO₂ and/or liquid CO₂. Embodiments of the present disclosure further include methods suitable for liquid natural gas production using the integrated multi-phase turbo expander and refrigeration means.

The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another and intended for the purpose of orienting the reader as to specific components parts. Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. The modifier “about” used in connection with a quantity is inclusive of the stated value, and has the meaning dictated by context, (e.g., includes the degree of error associated with measurement of the particular quantity). Accordingly, a value modified by a term or terms, such as “about”, is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value.

In the following specification and the claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. As used herein, the term “or” is not meant to be exclusive and refers to at least one of the referenced components being present and includes instances in which a combination of the referenced components may be present, unless the context clearly dictates otherwise. In addition, in this specification, the suffix “(s)” is usually intended to include both the singular and the plural of the term that it modifies, thereby including one or more of that term (e.g., “the jets” may include one or more jets, unless otherwise specified). Reference throughout the specification to “one embodiment,” “another embodiment,” “an embodiment,” and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the embodiment is included in at least one embodiment described herein, and may or may not be present in other embodiments. Similarly, reference to “a particular configuration” means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the configuration is included in at least one configuration described herein, and may or may not be present in other configurations. In addition, it is to be understood that the described inventive features may be combined in any suitable manner in the various embodiments and configurations.

As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable, or suitable. For example, in some circumstances, an event or capacity can be expected, while in other circumstances the event or capacity cannot occur—this distinction is captured by the terms “may” and “may be”.

Turning now to the drawings, FIG. 1 illustrates an exemplary liquid natural gas (LNG) production system 10 in accordance with one or more embodiments shown or described herein. The LNG production system 10 includes at least one cooling stage 200 configured to receive a natural gas stream 16. The system 10 further includes at least one compression stage 100 in fluid communication with the one or more cooling stage 200. The combination of the at least one compression stage 100 and the at least one cooling stage 200 is configured to compress and partially cool the natural gas stream 16 prior to expansion and further cooling of a low moisture compressed natural gas stream 21 in an expansion stage 300. A separation stage 400 in fluid communication with a multi-phase turbo expander (described presently) in the expansion stage 400 provides separation of a cold CH₄ vapor and a LNG/ice/solid CO₂ slurry, and further separation of the slurry into LNG and solid CO₂ components.

In a specific embodiment, the LNG production system 10 includes a bulk moisture removal device 12, in fluid communication with a natural gas inlet 14, and through which a natural gas (NG) stream 16 is fed. The moisture removal device 12 provides for preliminary moisture removal from the NG stream 16, to generate a low moisture NG stream 20 at an output of the moisture removal device 12. In an embodiment, the moisture removal device 18 may be configured as a molecular sieves bed. Other sorbent and solvent based systems known in the art may be also utilized.

The at least one cooling stage 200 includes one or more heat exchangers (described presently) disposed downstream of the moisture removal device 12 and in fluid communication therewith. During operation of the LNG production system 10, to avoid icing of the downstream heat exchanger(s), moisture in the NG stream 16 must be reduced in the moisture removal device 12. In contrast to the LNG production system 10 disclosed herein, in a conventional LNG liquefaction plant, a heat exchanger, typically operating at temperatures below −100° C. is utilized, requiring moisture in an input NG stream to be reduced to <0.5 ppm. In the present disclosure, the temperature of the NG stream 16 within in the heat exchanger is typically −40° C. or slightly higher, requiring moisture in the input NG stream 16 to be reduced to <180 ppm.

The compression stage 100 of the LNG production system 10 includes a first compressor 22, disposed between the moisture removals means 12 and one or more heat exchangers 24. More specifically, the first compressor 22 is disposed downstream of the moisture removal device 12, and upstream of the one or more heat exchangers 24. The first compressor 22 provides for the compression of the low moisture NG stream 20 to discharge a compressed NG stream 28. More particularly, in an embodiment the first compressor 22 discharges a low moisture compressed NG stream 21 at a pressure sufficient to provide for the reduction of the temperature in a final stage of cooling.

In an alternate embodiment, and as best illustrated in FIG. 2, the LNG production system may provide for moisture removal from the NG stream 16, subsequent to compression in the first compressor 22. More particularly, in an embodiment the compression stage 100 of the LNG production system includes a first compressor 22 disposed upstream of the moisture removals means 12 and the one or more heat exchangers 24. More specifically, in contrast to the embodiment of FIG. 1, the moisture removal device 12 is disposed downstream of the first compressor 22, and upstream of the one or more heat exchangers 24. In this particular embodiment, the first compressor 22 provides for the compression of the NG stream 16 to discharge the compressed NG stream 28. Subsequent removal of moisture from the compressed NG stream 28 discharges the low moisture NG stream 20, and more particularly the low moisture compressed gas stream 21.

Temperature of a gas typically increases as a result of the compression work. In an embodiment, the first compressor 22 comprises a multi-stage compressor 26 with inter stage cooling and provides compression of the low moisture NG stream 20. In an alternate embodiment, the first compressor 22 may comprise a plurality of compressors (not shown) having one or more air coolers disposed therebetween. The first compressor 22 generates the compressed NG air stream 28 at an outlet of the first compressor 22. NG cooling after the last compression stage of the multi-stage compressor 26 is achieved by an air cooler 29.

Referring again to FIG. 1, as indicated, the cooling stage 200 includes the one or more heat exchangers 24 (of which only one is illustrated) configured to remove heat from the low moisture compressed NG stream 21. The heat exchanger 24 is disposed downstream from the compression stage 100, and more particularly the first compressor 22. In an embodiment, the low moisture compressed NG stream 21 is pre-cooled in the one or more heat exchangers 24 to approximately −40° C. by cold methane vapor and configured to discharge a cooled compressed discharge stream 32. As indicated, in some embodiments, this cooling stage 200 may include a plurality of heat exchangers. It should be noted that in FIGS. 1 and 2, the single heat exchanger 24 is shown as an exemplary embodiment only and the actual number of heat exchangers and their individual configuration may vary depending on the end result desired. In some embodiments, the one or more heat exchangers 24 may be cooled using a cooling medium. In some embodiments, the one or more heat exchangers 24 may be cooled using cooling air, cooling water, or both. In some embodiments, the cooling stage 200 may further include one or more intercoolers (not shown) to cool the low moisture compressed NG stream 21 without substantially affecting the pressure.

The LNG production system 10, and more particularly the expansion stage 300 further includes a multi-phase turbo expander 30 configured to receive the cooled compressed discharge stream 32 from the heat exchanger 24 and generate an expanded exhaust stream 34. As noted earlier, the at least one cooling stage 200 is in fluid communication with the at least one expansion stage 300 including the multi-phase turbo expander 30, as described herein. As illustrated by a dotted line, the first compressor 22 and the multi-phase turbo expander 30 are typically mechanically coupled, such as through a common shaft 36. Alternatively, the multi-phase turbo expander 30 may be mechanically coupled with compressor 46. The temperature of the natural gas stream, and more particularly the cooled compressed discharge stream 32, is decreased during the expansion process primarily as a result of work extraction in the multi-phase turbo expander 30 with additional temperature reductions occurring through local depression of the static temperature in high velocity flow and the Joule-Thompson effect during throttling. During operation, energy recovered in the multi-phase turbo expander 30 is used to partially offset energy requirements for the compression stage 100. As the natural gas stream cools, it is partially converted to LNG and natural gas impurities such as moisture and CO₂ form solid phases. More particularly, the expansion process further cools the cooled compressed discharge stream 32 of natural gas generating the expanded exhaust stream 34 comprising a mixture of a vapor stream comprised substantially of CH₄ and a LNG/ice/solid CO₂ slurry.

In an embodiment, the moisture content in the NG stream 16, and more particularly the low moisture compressed NG stream 21, can be adjusted prior to expansion in the multi-phase turbo expander 30 to optimize the LNG output and CO₂ particle sizes using the bulk moisture removal system, and more particularly the moisture removal device 12, at the beginning of the process. In an embodiment, at least part of solid CO₂ and ice are separated from the NG stream 16 and removed from the multi-phase turbo expander 30 by forcing the swirl movement of the stream.

The LNG production system 10 further includes the at least one separation stage 400, including a separator 38, configured to receive the expanded exhaust stream 34 comprised of a mixture of a vapor stream comprised substantially of CH₄ and a LNG/ice/solid CO₂ slurry and separate the components into an output vapor stream 40 comprised substantially of CH₄ and a LNG/ice/CO₂ slurry stream 42. More particularly, the vapor stream 40 comprised substantially of CH₄ is separated from the LNG/ice/CO₂ slurry in the separator 38 and is recirculated in the LNG production system 10, as indicated by the arrows, to pre-cool the low moisture NG stream 20 prior to compression and expansion in the first compressor 22 and multi-phase turbo expander 30, respectively.

More particularly, the system 10 further includes a recirculation stage 500 configured to circulate at least a portion of the vapor stream 40 comprised substantially of CH₄ to the low moisture NG stream 20, and more particularly to the low moisture NG stream 20 upstream of the compression stage 100, and the compressor 22 in a recirculation path 501, as best illustrated in FIG. 1. In an alternate embodiment, the recirculation stage 500 is configured to circulate at least a portion of the vapor stream 40 comprised substantially of CH₄ to the compressed NG stream 28, and more particularly to the compressed NG stream 28 downstream of the compression stage 100, and the compressor 22 in a recirculation path 501, as best illustrated in FIG. 2. In some embodiments, the recuperation of cold CH₄ vapor stream 40 may result in additional cooling of the gas stream 16.

During the process of recirculation of the vapor stream 40 comprised substantially of CH₄, the vapor stream 40 is compressed in a second compressor 44. In an embodiment, the second compressor 44 is one of mechanically or electrically driven by a drive source 46 to provide for compression of the CH₄ vapor stream 40 and generate a compressed vapor stream 48.

The LNG/ice/solid CO₂ slurry stream 42 is separated in a liquid/solid separator 50 to form a liquid natural gas (LNG) output stream 52 and an output stream 54 comprised substantially of ice/solid CO₂. In an embodiment, the liquid/solid separator 50 is one of a gravity separator, a cyclone, a sintered metal filter or any type of filter configured to separate the solid contaminants from the LNG.

Referring now to FIG. 2, illustrated is an alternate embodiment of the LNG production system of the present disclosure. More particularly, illustrated is a LNG production system 60 including an integrated external refrigeration system. It should be understood that like elements have like numbers throughout the embodiments.

The LNG production system 60 includes a first compressor 22, in fluid communication with a natural gas inlet 14, and through which a natural gas (NG) stream 16 is fed. As described with regard to the previous embodiment, the first compressor 22 may comprise a multi-stage compressor 26 with inter stage cooling and provides compression of a NG stream 16. In an alternate embodiment, the first compressor 22 may comprise a plurality of compressors (not shown) having one or more air coolers disposed therebetween. The first compressor 22 generates a compressed NG air stream 28 at an outlet of the first compressor 22. The LNG production system 60 further includes a moisture removal device 12, disposed between the first compressor 22 and a heat exchanger 24. A heat exchanger 24 is disposed downstream of the moisture removal device 12 and in fluid communication therewith. More specifically, the moisture removal device 12 is disposed downstream of the first compressor 22, and upstream of the heat exchanger 24. The heat exchanger 24 is utilized to remove heat from the low moisture compressed NG stream 21. The heat exchanger 24 is configured to discharge a cooled compressed discharge stream 32. The LNG production system 60 further includes a multi-phase turbo expander 30 configured to receive the cooled compressed discharge stream 32 from the heat exchanger 24 and generate an expanded exhaust stream 34. The first compressor 22 and the multi-phase turbo expander 30 are typically mechanically coupled, such as through a common shaft 36. The expansion process further cools the cooled compressed discharge stream 32 of natural gas generating the expanded exhaust stream 34 comprising a mixture of a vapor stream comprised substantially of CH₄ and a LNG/ice/solid CO₂ slurry.

The LNG production system 60 further includes a separator 38, configured to receive the expanded exhaust stream 34 comprised of a mixture of a vapor stream comprised substantially of CH₄ and a LNG/ice/solid CO₂ slurry and separate the components into a vapor stream 40 comprised substantially of CH₄ and a LNG/ice/CO₂ slurry stream 42. As previously described, the cold vapor CH₄ is separated from the LNG/ice/CO₂ slurry in the separator 38 and is recirculated in the LNG production system 60, as indicated by the arrows, to pre-cool the compressed NG stream 28 prior to removal of moisture, cooling and expansion in the moisture removal device 12, the heat exchanger 24 and the multi-phase turbo expander 30, respectively.

During the process of recirculation of the vapor stream 40 comprised substantially of CH₄, the vapor stream 40 is compressed in a second compressor 44 to generate a compressed vapor stream 48 comprised substantially of CH₄. The LNG/ice/solid CO₂ slurry stream 42 is separated in a liquid/solid separator 50 to form a liquid natural gas (LNG) output stream 52 and an output stream 54 comprised of substantially ice/solid CO2.

In contrast to the embodiment of FIG. 1, the embodiment illustrated in FIG. 2 further includes an external refrigeration system 62 configured to generate a cold vapor stream 64. The external refrigeration system 62 is configured in fluidic communication with the heat exchanger 24. During operation, the external refrigeration system 62 provides pre-cooling in the heat exchanger 24 to approximately −40° C. prior to expansion in the multi-phase turbo expander 30. The cold vapor stream 64 provides further cooling of the discharge stream 32 of up to −60° C. As previously indicated, although the external refrigeration system 62 is not required, use of such additional refrigeration means provides additional energy to the heat exchanger 24 and further cooling of the low moisture compressed NG stream 21. In an embodiment, the external refrigeration system 62 is configured as a propane refrigeration system.

In the disclosed embodiments, as shown in FIGS. 1 and 2, a multi-phase turbo expander, and more particularly the multi-phase turbo expander 30, for expanding the cooled compressed discharge stream 32 generated by the heat exchanger 42 is presented. Expansion of the cooled compressed discharge stream 32 generates the expanded exhaust stream 34 comprising the mixture of cold CH₄ vapor and the LNG/ice/solid CO₂ slurry. The term “multi-phase turbo expander” as used herein refers to a radial, axial, or mixed flow turbo-machine through which a gas or gas mixture is expanded to produce work and additional output components.

Referring now to FIG. 3, illustrated in a simplified block diagram is a multi-phase turbo expander 70, generally similar to the multi-phase turbo expander 30 of FIGS. 1 and 2, in accordance with embodiments of the disclosure. FIG. 4 illustrates in a cross-sectional view, an embodiment of the multi-phase turbo expander 70, in accordance with an embodiment of the disclosure. As indicated in FIG. 4, the multi-phase turbo expander 70 includes a housing 72. As indicated in FIG. 3, the multi-phase turbo expander 70 further includes at least one rotating component or a rotor 74 configured to extract work from a flow stream. In some embodiments, the multi-phase turbo expander 70 further includes at least one stationary component 76. The stationary component 76 may include a stator or a nozzle. In some embodiments, the nozzle is equipped with vanes that force swirl stream movement. As indicated in FIG. 4, the multi-phase turbo expander 70 further includes one or more seals 78, in some embodiments. As indicated in FIG. 4, the multi-phase turbo expander 70 further includes one or more blades 80/82. In some embodiments, the multi-phase turbo expander 70 further includes one or more stationary blades 80 and one or more rotor blades 82, as indicated in FIG. 4.

As indicated in FIGS. 3 and 4, the multi-phase turbo expander 70 further includes at least one inlet 84 disposed in the housing 72. The inlet 84 is configured to receive a gas stream 86, in some embodiments. The gas stream 86 is generally similar to the cooled compressed discharge stream 32 of FIGS. 1 and 2 exiting the heat exchanger 24, subsequent to compression in a compressor, such as the first compressor 22 of FIGS. 1 and 2 and cooling in the heat exchanger 24. The multi-phase turbo expander 70 further includes at least one outlet 88 disposed in the housing 72. The outlet 88 is configured to discharge an expanded exhaust stream 90, in some embodiments. The expanded exhaust stream 90 is generally similar to the expanded exhaust stream 34 of FIGS. 1 and 2, exiting the multi-phase turbo expander 70, subsequent to expansion in the multi-phase turbo expander 70, such as the multi-phase turbo expander 30 of FIGS. 1 and 2. In an embodiment, the multi-phase turbo expander 70 may include additional outlets and/or separation means (described presently).

As noted earlier, the gas stream 86 includes carbon dioxide and having a specific moisture content. In some embodiments, the gas stream 86 further includes one more of nitrogen, heavy hydrocarbons, or water vapor. In some embodiments, the gas stream 86 further includes impurities, example of which include, but are not limited to, hydrogen sulfide. In some embodiments, the gas stream 86 is substantially free of the impurities. In some embodiments, the gas stream 86 includes nitrogen and carbon dioxide.

In some embodiments, the amount of impurities in the gas stream 86 is less than about 50 mole percent. In some embodiments, the amount of impurities in the gas stream 86 is less than about 20 mole percent. In some embodiments, the amount of impurities in the gas stream 86 is in a range from about 10 mole percent to about 20 mole percent. In some embodiments, the amount of impurities in the gas stream 86 is less than about 5 mole percent.

As noted earlier, the gas stream 86 expands in the multi-phase turbo expander 70 and as the work is extracted from the expanding gas stream, the gas stream 86 is cooled inside the multi-phase turbo expander 70. Cooling the gas stream 86 in the multi-phase turbo expander 70 results in formation of a CH₄ vapor, LNG, and one or both of solid CO₂ and liquid CO₂ in the multi-phase turbo expander 70. More specifically, the multi-phase turbo expander 70 is configured to cool the gas stream 86 such that the gas stream 86 primarily forms a vapor stream comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry stream, generally similar to streams 40 and 42 of FIGS. 1 and 2. In an embodiment, the term “primarily forms” as used herein means that the amount of solid CO₂ formed in the multi-phase turbo expander is less than about 5 mass percent. In an embodiment, the term “primarily forms” as used herein means that the amount of LNG formed in the multi-phase turbo expander is less than about 45 mass percent. In an embodiment, the term “primarily forms” as used herein means that the amount of CH₄ vapor formed in the multi-phase turbo expander is less than about 50 mass percent.

In some embodiments, a flow field within the multi-phase turbo expander 70 may be utilized to aid in separation of the LNG/ice/solid CO₂ slurry and the CH₄ vapor. This may be accomplished by incorporating one or more separation channels (not shown) into the multi-phase turbo expander housing 72 and additional outlets. In some embodiments, the separation channels may be designed such that the liquid or solid particles enter due to centrifugal force and may be precluded from re-entering the multi-phase turbo expander flow path by a deflector. Example configurations for the inclusion of certain aspects of the multi-phase turbo expander 70, such as multiple outlets, separation channels, and the like, are described in commonly assigned, U.S. Publication No. 2013/0125580, D. Hofer, “Expander and Method of CO₂ Separation”, which is incorporated by reference herein in its entirety.

In some embodiments, at least one component of the multi-phase turbo expander 70 further includes a coating configured to preclude adhesion of solid CO₂ to a surface of the multi-phase turbo expander component. In some embodiments, one or more of the housing 72, the rotating component 74, or the stationary component 76 may include a coating configured to preclude adhesion of solid CO₂ to a surface of the multi-phase turbo expander component. In particular embodiments, the rotating component 74 in the multi-phase turbo expander 70 includes a coating 92. In some embodiments, the coating 92 is configured to preclude adhesion of solid CO₂ to a surface 94 of the rotating component 74. In some embodiments, the coating 92 includes a non-stick material capable of precluding adhesion of solid CO₂ to the surface 94 of the rotating component 74.

In some embodiments, the multi-phase turbo expander 70 further includes at least one heated component. In some embodiments, the heated component is configured to preclude adhesion of solid CO₂ to a surface of the multi-phase turbo expander component. In some embodiments, one or more of the housing 72, the rotating component 74, or the stationary component 76 may include a heated component to preclude adhesion of solid CO₂ to a surface of the multi-phase turbo expander component. In particular embodiments, a stationary component 76 in the multi-phase turbo expander 70 is heated to preclude adhesion of solid CO₂ to a surface 96 of the stationary component 76.

In some embodiments, one or more of the stationary blades 80 may be heated by using electrical heating elements. In such an embodiment, the blade 80 may include heated elements (not shown) disposed in one or more holes formed in the blade 80. In some embodiments, one or more components of the multi-phase turbo expander 70 may be heated by circulating air or gas. The blade 80, in some embodiments may further include gas flow channels (not shown), such as, for example, Z-shaped channels. In some embodiments, the gas flow channels may have any suitable shape, such as, for example, U-shape, E-shape, and the like. As previously indicated, example configurations for the inclusion of certain aspects of the multi-phase turbo expander 70, including the use of electrical heating elements, circulating air or gas to heat the blades, gas flow channels, and the like, are described in commonly assigned, U.S. Publication No. 2013/0125580, D. Hofer, “Expander and Method of CO₂ Separation”, which as previously noted, is incorporated by reference herein in its entirety.

The multi-phase turbo expander configuration, in accordance with some embodiments of the disclosure may advantageously allow for separation of a CH₄ vapor stream and a liquid natural gas/ice/solid CO₂ slurry stream from the gas stream within the multi-phase turbo expander itself, thus precluding the need for an additional separator, such as separator 38 of FIGS. 1 and 2.

As indicated in FIGS. 3 and 4, the multi-phase turbo expander 70 further includes the at least one outlet 88 configured to discharge the expanded exhaust stream 90 comprised of the CH₄ vapor, and the LNG/ice/solid CO₂ slurry. As indicated in FIGS. 3 and 4, in some embodiments, the at least one outlet 88 is disposed downstream of the rotating component 74. In some embodiments, the expanded exhaust stream 90 may include one or more non-condensable components. In some embodiments, the expanded exhaust stream 90 may include one or more liquid components. In some embodiments, the expanded exhaust stream 90 may include one or more solid components. The expanded exhaust stream 90 may be further configured to be in fluid communication with one or both of a liquid-gas and a solid-gas separator, such as separators 38 and 50 disclosed in FIGS. 1 and 2.

In some embodiments, the multi-phase turbo expander 70 for generating LNG from a gas stream 86 may include a single-stage multi-phase turbo expander, as illustrated in FIGS. 3 and 4. In some other embodiments, the multi-phase turbo expander 70 for generating LNG from a gas stream 86 may include a multi-stage multi-phase turbo expander 70, as described in commonly assigned, U.S. Publication No. 2013/0125580, D. Hofer, “Expander and Method of CO₂ Separation”, incorporated by reference herein in its entirety.

Turning now to FIG. 5, in an embodiment, a method 600 for generating LNG from a gas stream is provided. The method includes processing an input gas stream in a cooling stage, compression stage, expansion stage and separation stage. More specifically, the method includes the input of a gas stream, and more particularly a natural gas stream, in a step 602. The method further includes compressing the input gas stream in a compression stage, in a step 604. The compressed gas stream is next expanded, in a step 606, in a multi-phase turbo expander in an expansion stage. The expansion stage produces energy and an expanded exhaust stream comprised of a mixture of cold CH₄ vapor and a LNG/ice/solid CO₂ slurry. Next, in a step 608, the expanded exhaust stream is separated in a separator into components, and more specifically, into an output cold CH₄ vapor stream and a LNG/ice/CO₂ slurry stream. The CH₄ vapor stream is recirculated, in a step 610, to the input gas stream, to pre-cool the gas stream prior to compression and expansion. In addition, the LNG/ice/solid CO₂ slurry stream is further separated in a separator to discharge, in a step 612, a liquid natural gas stream and a solid CO₂ stream.

Referring now to FIG. 6, illustrated in an exemplary graphical representation, generally referenced 700, are comparative compression ratios achieved during the previously described compression stage and process energy requirements, in accordance with an exemplary embodiment. More specifically, graph 700 illustrates compression ratios (plotted in axis 702) with energy requirements per gallon of LNG output stream (plotted in axis 704) of a novel liquid natural gas (LNG) production system, in accordance with an embodiment described herein. The energy requirements per gallon of LNG output stream (shown by plotted points/line 706), illustrates as the compression ration increases, the energy requirement decreases until it reaches a steady level, near 706, at compression ratios above approximately 30 psi. More specifically, illustrated in FIG. 6 is that normalized by the amount of LNG produced, process energy requirements decrease as the compression ratio increases and then level off at approximately 0.8 kWh/gallon at compression ratio of ˜30.

An initial NG pressure of 10 atm (corresponding to stream 16 in FIGS. 1 and 2) was used to calculate compression ratios shown in FIG. 6. As the initial pressure changes, shape of the compression curve in FIG. 6 also changes.

Accordingly, disclosed is a novel method and system for the production of liquid natural gas that integrates natural gas (NG) cleanup and refrigeration steps to produce the LNG in one process step by utilizing a multi-phase turbo expander. The method and system include the multi-phase turbo expander capable of operation with multi-phase flows (gas, liquids, and solids) and wherein a mixture of a cold CH₄ vapor and a LNG/ice/solid CO₂ slurry are discharged. The resulting system and method provide lower equipment cost, smaller footprint, increased LNG production and lower overall cost than conventional LNG production processes.

This written description uses examples to disclose the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

What is claimed is:
 1. A system for producing liquid natural gas (LNG) from a natural gas stream, comprising: a moisture removal device and a compressor for removing moisture from and compressing the natural gas stream and generating a low moisture compressed natural gas stream; a heat exchanger for cooling the low moisture compressed natural gas stream and generating a cooled compressed discharge stream; a multi-phase turbo expander for expanding the cooled compressed discharge stream and generating an expanded exhaust stream comprised of a mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry; a separator for separating the expanded exhaust stream and generating a vapor stream comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry stream; and at least one additional separator for separating the liquid natural gas/ice/solid CO₂ slurry stream and generating a liquid natural gas output stream and an output stream comprised substantially of ice/solid CO₂.
 2. The system of claim 1, wherein the moisture removal device comprises molecular sieves beds.
 3. The system of claim 1, wherein the low moisture compressed natural gas stream comprises a natural gas stream having a moisture content of less than 180 ppm.
 4. The system of claim 1, wherein the multi-phase turbo expander is configured to further cool the cooled compressed discharge stream from the heat exchanger such that primarily formed is the liquid natural gas/ice/solid CO₂ slurry.
 5. The system of claim 1, further comprising a recirculation path configured to recirculate the vapor stream comprised substantially of CH₄ into the natural gas stream.
 6. The system of claim 5, wherein the recirculation path further comprises a compressor for compressing the vapor stream comprised substantially of CH₄ and generating a compressed vapor stream comprised substantially of CH₄.
 7. The system of claim 1, wherein the at least one additional separator is one of a filter, a cyclone or a gravity separator.
 8. The system of claim 1, wherein the multi-phase turbo expander comprises: a housing; at least one rotating component disposed within the housing; at least one inlet disposed in the housing, wherein the inlet is configured to receive the cooled compressed discharge stream; and at least one outlet disposed in the housing, wherein the outlet is configured to discharge the expanded exhaust stream comprised of a mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry, wherein the multi-phase turbo expander is configured to further cool the cooled compressed discharge stream from the heat exchanger such that the vapor comprised substantially of CH₄ is formed, a liquid natural gas is formed and at least a portion of CO₂ in the cooled discharge natural gas stream forms a solid CO₂.
 9. The system of claim 8, wherein the multi-phase turbo expander is further configured to separate the vapor comprised substantially of CH₄ and the liquid natural gas/ice/solid CO₂ slurry to discharge a vapor stream comprised substantially of CH₄ and a slurry stream comprised substantially of a liquid natural gas/ice/solid CO₂.
 10. The system of claim 1, further comprising an external refrigeration system in fluidic communication with the heat exchanger to provide further cooling to the low moisture compressed natural gas stream.
 11. A system for producing liquid natural gas (LNG) from a natural gas stream, comprising: at least one compression stage and at least one cooling stage configured to compress and cool a natural gas stream and generate a cooled compressed discharge stream; and at least one expansion stage configured to expand the cooled compressed discharge stream and generate an expanded exhaust stream, the at least one expansion stage comprising at least one multi-phase turbo expander in fluid communication with the compression stage and the cooling stage, the multi-phase turbo expander comprising: a housing; at least one rotating component disposed within the housing; at least one inlet disposed in the housing, wherein the inlet is configured to receive the cooled compressed discharge stream; and at least one outlet disposed in the housing, wherein the outlet is configured to discharge an expanded exhaust stream comprising a mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry.
 12. The system of claim 11, further comprising a separation stage configured to separate the mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry into a vapor stream comprised substantially of CH₄ and an output stream comprised substantially of liquid natural gas/ice/solid CO₂ slurry.
 13. The system of claim 12, further comprising a recirculation stage configured to recirculate the vapor stream comprised substantially of CH₄ into the natural gas stream.
 14. The system of claim 13, wherein the recirculation stage comprises a compressor for compressing the vapor stream comprised substantially of CH₄ and generating a compressed vapor stream comprised substantially of CH₄.
 15. The system of claim 11, wherein the multi-phase turbo expander is configured to further cool the cooled compressed discharge stream such that a liquid natural gas is formed and at least a portion of CO₂ in the cooled discharge natural gas stream forms a solid CO₂.
 16. The system of claim 11, at least one cooling stage further comprises an external refrigeration system configured to provide further cooling to the low moisture compressed natural gas stream.
 17. A method for producing liquid natural gas (LNG) from a natural gas stream, comprising: providing an input natural gas stream; removing moisture, compressing and cooling the natural gas stream and generating a cooled compressed discharge stream; expanding the cooled compressed discharge stream in a multi-phase turbo expander and generating an expanded exhaust stream comprising a mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry; separating the expanded exhaust stream in at least one separator and generating a vapor stream comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry stream; and separating the liquid natural gas/ice/solid CO₂ slurry stream in at least one additional separator and generating an output stream comprised substantially of ice/solid CO₂ and a liquid natural gas (LNG) output stream.
 18. The method of claim 17, further comprising recirculating the vapor stream comprised substantially of CH₄ to the input natural gas stream in a recirculation path.
 19. The method of claim 17, further comprising separating at least portion of ice/solid CO₂ from the cooled compressed natural gas stream in the multi-phase turbo expander.
 20. The method of claim 17, wherein the multi-phase turbo expander comprises: a housing; at least one rotating component disposed within the housing; at least one inlet disposed in the housing, wherein the inlet is configured to receive the cooled compressed discharge stream; and at least one outlet disposed in the housing, wherein the outlet is configured to discharge the expanded exhaust stream comprising a mixture of a vapor comprised substantially of CH₄ and a liquid natural gas/ice/solid CO₂ slurry; wherein the multi-phase turbo expander is configured to further cool the cooled compressed discharge stream such that the vapor comprised substantially of CH₄ is formed, a liquid natural gas is formed and at least a portion of CO₂ in the cooled discharge natural gas stream forms a solid CO₂.
 21. The method of claim 17, further comprising further cooling the natural gas stream in an external refrigeration system in fluidic communication with the cooled compressed discharge stream.
 22. The method of claim 17, further comprising separating at least portion of the vapor stream comprised substantially of CH₄ and a slurry stream comprised substantially of a liquid natural gas/ice/solid CO₂ within the multi-phase turbo expander. 